Thermostatic devices have previously been disclosed, the lower portion of which is in the form of a drawn or pressed cup. This cup may be electrically conducting or may be insulating, depending upon the construction. A bimetallic element is attached to some portion of this cup to provide the operating mechanism of the thermostatic device. The opening in the cup is then closed by attaching a lid of some type. Again, the lid may be electrically conducting or insulating.
Particularly when the cup and lid are formed of electrically conducting materials, prior to final assembly of the device, a sheet of insulating material is generally placed over a ledge which is formed ont he cup member, the lid is then placed above this, and the side edges of the cup are then rolled over so as to crimp the lid and insulating member in position. Structures of this type are shown, for example, in U.S. Pat. No. 3,100,827, Grimshaw; U.S. Pat. No. 3,430,177, Audette; and U.S. Pat. No. 4,521,760, Carbone et al.
As illustrated in the referenced patents, it is generally the sides of the cup-shaped members which are rolled over in order to crimp the lid and the insulating sheet to the cup shaped member, and no crimp is provided at the ends of the device. Frequently, the thermostatic device is employed in a motor winding which is varnish impregnated after completion of winding. When the thermostatic device as just described is employed, because of the lack of crimping at the ends, the varnish, prior to drying, frequently leaks into the thermostatic device through the ends and may either disable the device, entirely, or, at least, distort its planned operation.
Since the crimping of a lower cup shaped member to an upper lid, with the intermediate insulating member, is such a desirable method of operation, particularly for reasons of economy, means have been sought to prevent the leakage of insulating materials into thermostatic devices formed in this way, but without particular success. For example, the insulating sheet which is placed between the cup and the lid is frequently impregnated with a B stage resin which cures upon heating of the device. Similarly, attempts have been made to seal the ends of the device with Glyptal, an alkyd-type polymer of the General Electric Company, and other similar polymers. The B stage resin impregnated insulators are ineffective, and, while the Glyptal seal prevents insulation material from leaking through, the Glyptal, itself, leaks through to create the same problems as the insulating varnish.